Evil remaining after good man departs (3)
I believe the answer is:
ill
'evil' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for ill that I've seen before include "Not in good health" , "Unhealthy" , "Faulty" , "Badly diminished" , "Badly - in need of treatment" .)